史论结合是学习历史的重要方法。下列史实与结论搭配正确的一组是（    ）

将2个相同的a和2个相同的b共4个字母填在$3\times 3$的方格内，每个小方格内至多填1个字母，若使相同字母既不同行也不同列，则不同的填法种数为          （  ）

D

    Jack is a 13-year-old boy.

    One day, his father asks him, "How many letters are there in the alphabet(字母表)?"

    "24." He says.

    "24?" His father asks loudly, "Are you sure?"

    "Sorry, I don't know." Answers the son.

    "You don't know? You are in school for 7 years and you don't know how many letters there are in the alphabet?" The father is very angry(生气的).

    His son says, "No. But let me ask you a question, dad. You often go to the post office(邮局). Please tell me how many letters there are in the post office."